We just took a 27 hour trip from Connecticut to Florida wiht a 2 year old, 3 year old and a 4 year old. I have been making this trip since the 4 year old was a newborn.
The way we do things is: Leave the house at 3am. They stay awake about an hour and then sleep from about 4am to8am. Get out, change them into clothes, eat a long breakfast and get back in the car for another "longer" 3 hour stretch. Get out, RUN the kids (have them go back and forth around tree's and play catch etc with them) when they are tired get them back in the car for another 2 or so hours. Get out, run them, eat lunch. By then they should be tired enough to zone in front of a DVD player for a couple of hours. Get them out of the car after 2 hours, run them, get back in the car and have them fall asleep till dinner. Eat for an hour, run them and get back in the car until you and them can't take it anymore.
By this time you have gone about18 hours. Spend the night in a hotel and then the next day stop about every hour and a half until you get to your destination. It's the second day that you will really need your patience... and the DVD player. Everyone is just DONE at that point.
Hope this helps!